The present invention relates to filtering of radio frequency signals, and more particularly to digital techniques for filtering radio frequency signals that are supplied in log-polar format.
In the telecommunications arts, such as in mobile telephony, it is known that it is always possible to represent an arbitrary radio signal as a sequence of composite (complex) vectors. Thus, a radio signal can be expressed either in Cartesian (I, Q) form or in polar (RSS, PHI) form, where RSS is the received signal strength, and PHI represents the phase angle of the vector. It is also known that a so-called "log polar form" can be advantageously used as an alternative to the two forms mentioned above.
FIG. 1 is a block diagram of a conventional log polar receiver. A radio signal is received by an antenna 101 and supplied to an amplifier 103. The amplified signal is then supplied to a mixer 105, where it is mixed with a signal generated by a local oscillator 107 to produce a signal having a suitable intermediate frequency ("I.F. signal"). The I.F. signal is then supplied to a bandpass filter whose purpose is to pass only those frequencies that lie within the range of a bandwidth centered around a predefined center frequency.
After further amplification by amplifier 111, the analog I.F. signal 113 is supplied to a log polar digitizer 127. In a first leg of the log polar digitizer 127, the analog I.F. signal 113 is amplified by a logarithmic amplifier 115 and then converted to a digital form by the analog-to-digital (A/D) converter 117. Each output of the A/D converter 117 represents the log of the received signal strength (rss 119) at a particular instant in time.
In another leg of the log polar digitizer 17, the analog I.F. signal 113 is supplied to a phase digitizer 121, which generates a digital signal, PHI 123, which represents the phase of the applied analog I.F. signal 113.
The digital signals rss 119 and PHI 123, which are generated by the log polar digitizer 127, are then supplied to a demodulator 125 which processes these signals using known digital techniques to generate a demodulated signal.
The performance of the bandpass filter 109 is important because it determines the extent to which the receiver will respond to all frequencies within the defined channel, and reject (i.e., not respond to) all frequencies falling outside the channel. FIG. 2 is a graph of the frequency characteristics of the bandpass filter 109. The bandpass filter 109 is designed to pass only those frequency components of the input signal that lie in the range from f.sub.CENTER -A to f.sub.CENTER +B. In the illustrated example, f.sub.CENTER is selected to be a desired I.F. for the receiver circuit.
In the conventional receiver, the bandpass filter 109 is constructed entirely from analog components. This introduces a number of problems due to imperfections and variations of components during construction as well as variations that arise as a result of aging of the components. For example, referring to FIG. 2, the filter's bandwidth, A+B, may be too wide. This results in signals from adjacent channels being passed on to the demodulator. or, the filter's bandwidth, A+B, may be too narrow. This results in loss of performance at the desired channel due to parts of the desired signal being removed. Also, it is possible that the center frequency, F.sub.CENTER, is incorrect, so that A.noteq.B. In this case, parts of the desired signal will be removed and parts of an adjacent channel will be introduced.